Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. The various applications of fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs, Wiley Science 1997.
The known fluoropolymers include in particular fluoroelastomers and fluorothermoplasts. Such fluoropolymers include copolymers of a gaseous fluorinated olefin such as tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and/or vinylidene fluoride (VDF) with one or more comonomers such as for example hexafluoropropylene (HFP) or perfluorovinyl ethers (PVE) or non-fluorinated olefins such as ethylene (E) and propylene (P). The term “copolymer” in connection with the present invention should generally be understood to mean a polymer comprising repeating units derived from the recited monomers without excluding the option of other further repeating units being present that derive from other monomers not explicitly recited. Accordingly, for example the term ‘copolymer of monomers A and B’ includes binary polymers of A and B as well as polymers that have further monomers other than A and B such as for example terpolymers.
Examples of fluoroelastomers include for example copolymers of TFE and PVE and copolymers of VDF and HFP. The fluoroelastomers may also contain cure site components so that they may be cured if desired. Applications of fluoroelastomers include for example coatings, use as gaskets and seals as well as use as polymer processing aids (PPA). A commercially available processing aid includes for example copolymers of VDF and HFP available from Dyneon LLC under the brand DYNAMAR™ PPA.
Examples of fluorothermoplasts include copolymers of TFE and E(ETFE), copolymers of TFE and HFP (FEP), copolymers of TFE, HFP and VDF (THV) and perfluoroalkoxy copolymers (PFA). Examples of applications of fluorothermoplasts include for example coating applications such as for example for coating outdoor fabric and use as insulating material in wire and cable insulation. In particular ETFE copolymers have desirable properties as insulating material. Further applications of fluorothermoplasts include making of tubes such as for example fuel hoses, extrusion of films and injection molded articles. The extruded fluorothermoplastic articles, in particular films may further be subjected to an e-beam radiation to partially cure the fluorothermoplast.
Several methods are known to produce the fluoropolymers. Such methods include suspension polymerization as disclosed in e.g. U.S. Pat. No. 3,855,191, U.S. Pat. No. 4,439,385 and EP 649863; aqueous emulsion polymerization as disclosed in e.g. U.S. Pat. No. 3,635,926 and U.S. Pat. No. 4,262,101; solution polymerization as disclosed in U.S. Pat. No. 3,642,742, U.S. Pat. No. 4,588,796 and U.S. Pat. No. 5,663,255; polymerization using supercritical CO2 as disclosed in JP 46011031 and EP 964009 and polymerization in the gas phase as disclosed in U.S. Pat. No. 4,861,845.
Currently, the most commonly employed polymerization methods include suspension polymerization and especially aqueous emulsion polymerization. Aqueous emulsion polymerization has been generally preferred for the production of fluoropolymers because the process is more environmental friendly than solution polymerization in organic solvents and furthermore allows for easy recovery of the resulting polymer. However, for certain applications, the fluoropolymers produced via the aqueous emulsion polymerization process may have somewhat inferior properties relative to similar polymers produced via solution polymerization. For example, copolymers of E and TFE produced according to the solution polymerization disclosed in U.S. Pat. No. 4,123,602 generally have a better heat resistance than similar polymers produced via aqueous emulsion polymerization.
The aqueous emulsion polymerization normally involves the polymerization in the presence of a fluorinated surfactant, which is generally used to ensure the stabilisation of the polymer particles formed. The suspension polymerization generally does not involve the use of surfactant but results in substantially larger polymer particles than in case of the aqueous emulsion polymerization. Thus, the polymer particles in case of suspension polymerization will quickly settle out whereas in case of dispersions obtained in emulsion polymerization generally display good latex stability over a long period of time.
Although the aqueous emulsion polymerization generally involves the use of a fluorinated surfactant; there is also known an aqueous emulsion polymerization process wherein no fluorinated surfactant is added to the polymerization system. Such a polymerization is described in for example U.S. Pat. No. 5,453,477 and WO 97/17381. It is taught therein that through careful selection of the initiation system, the polymer particles are self-stabilized through the ionic endgroups of the polymers produced in the process.
To tailor the molecular weight of the resulting fluoropolymer, it has been known to use chain transfer agents. For example, U.S. Pat. No. 4,766,190 discloses an aqueous emulsion polymerization to make fluoroelastomers. The chain transfer agents disclosed include C4-C6 hydrocarbons, alcohols, ethers, esters, ketones and halocarbons.
U.S. Pat. No. 4,262,101 also discloses an aqueous emulsion polymerization process. Chain transfer agents used here include halocarbons, aliphatic carboxylic acid esters, ketones, alcohols, malonic esters and lower dialkylglycol.
U.S. Pat. No. 5,608,022 discloses the preparation of a copolymer of TFE and a comonomer of the formula CF2═CF—O—(CF2CF(CF3)—O)n—(CF2)m—Z wherein n is 1 or 2, m is 24 and Z is CO2R or SO2F with R being C1-C3 alkyl. The polymerization is carried out by dispersing the comonomer in water to a droplet size of less than 2 mm and copolymerizing with TFE in the presence of a C1-C6 alcohol or a C2-C6 ether. The alcohols are being disclosed as the preferred chain transfer agent and the chain transfer agents are being mixed as liquid with the aqueous polymerization medium. It is disclosed that the copolymer can be produced avoiding formation of different kind of polymers in the aqueous medium, in particular the formation of homopolymer of TFE is avoided. In the examples, n-propyl ether is used as a chain transfer agent. However ethers such as n-propyl ether and diethyl ether have been found to cause building of hazardous peroxides which may present a safety issue when used on an industrial scale to manufacture fluoropolymers. The polymers disclosed in this US patent are used as a base material of a salt electrolytic cation-exchanged film.
Alkanes have also been disclosed as chain transfer agents in an aqueous emulsion polymerization. For example, U.S. Pat. No. 3,635,926 discloses the use methane or ethane to make copolymers of tetrafluoroethylene and perfluorovinyl ethers.
WO 00/32655 discloses the use of hydrofluoroethers (HFEs) for the fluoromonomer emulsion-polymerization. These HFEs are taught to be superior to common chain transfer agents such as chloroform, in being safer and environmentally benign. These liquid components are also taught to be superior to gaseous chain transfer agents such as ethane because they do not enter the fluoromonomer gas recycle streams. However, the chain transfer activity of HFEs is fairly low, making them generally only practical in producing perfluorinated polymers where only a low chain transfer activity is desired and needed. When using the HFEs in producing partially fluorinated fluoropolymers, one will generally need a large amount of chain transfer, which is undesirable.
In JP 1-129005 there is disclosed the use of dialkyl ether chain transfer agents that have not more than 6 carbon atoms in a suspension polymerization of vinylidene fluoride to produce homo- or copolymers of vinylidene fluoride. Specifically disclosed ethers include dimethyl ether and diethyl ether with the latter being preferred. The polymerization temperature is taught to be between 10 and 25° C. It is disclosed that the use of these chain transfer agents allow for control of molecular weight of the vinylidene fluoride polymer without substantially affecting the polymerization rate and the heat resistance of the polymer produced.
A commonly employed chain transfer agent in the production of fluorothermoplasts and fluoroelastomers is diethylmalonate. The use of diethylmalonate is for example recommended in EP 43 948 to produce copolymers of TFE and E, such as for example copolymers of TFE, E, HFP and PVE. However, it has been found that fluoropolymers produced in the presence of this chain transfer agent are susceptible to discoloration, may produce an unpleasant smell and have a high amount of extractable compounds. Also, the fluoropolymers so produced have been found to have a large amount of low molecular weight fraction which causes processing difficulties of the fluoropolymer. Furthermore, the yield of these polymers when produced through aqueous emulsion polymerization in the presence of diethylmalonate would desirably be improved.
Further known chain transfer agents used in aqueous emulsion polymerization include silanes as disclosed in U.S. Pat. No. 5,256,745 and U.S. Pat. No. 5,208,305. However, also in this instance, it was found that the fluoropolymers produced have undesirable properties such as discoloration and low purity, in particular high amounts of extractable compounds. Additionally, the process disclosed in these patents to; produce a bimodal molecular weight distribution of the fluoropolymer is cumbersome, e.g., the polymerization time is long and the polymerization initiation is often retarded.
It would thus be desirable to improve the aqueous emulsion polymerization process so as to produce fluoropolymers with improved properties. It is in particular a desire to produce fluoropolymers that have a high purity, less extractable compounds, less smell, improved processing and less discoloration. It is further desirable to produce partially fluorinated fluoroelastomers and fluorothermoplasts that have improved mechanical and physical properties. Desirably, the chain transfer agents have a high chain transfer activity such that they can be used in low amounts.